1. Field of the Invention
The invention relates to a method for making an optoelectronic amplifier device, and a device obtained by this method. In particular, it relates to a device including an optical amplifier monolithically integrated with a waveguide, as well as to its manufacturing method. It also relates to the application of an amplifier such as this to optoelectronic devices such as change-over switches, modulators and distributors.
2. Description of the Prior Art
For the past few years, an increasing amount of research has been devoted to optical amplifiers, made either on doped optical fibers or on semiconductor materials.
As for semiconductor optical amplifiers, several applications are envisaged for telecommunications by optical fibers. These amplifiers could be used:
either at transmission, as boosters, PA1 or in line to advantageously replace standard repeaters and thus increase the range of the link; PA1 or again at reception as preamplifiers. PA1 N. A. OLSSON et al.: "Polarization Independent Optical Amplifier With Buried Facets", Electronics Letters, Vol. 25, No. 16, p. 1048-1049, Aug. 3, 1989. PA1 I. CHA et a.: "1.5 Band Travelling Wave Semiconductor With Window Facet Structure", I00C'89 (Proceedings), Jul. 18-21, 1989, Kobe, Japan. PA1 a) a stage for the epitaxy, on a substrate, of the following layers: PA1 b) A first step for etching at least one amplifier element in the active layer, the confinement layer and the contact layer; PA1 c) a second step for etching at least one optical guide located beneath the amplifier element, this etching being done in the second optical guiding layer; PA1 d) a step for the deposition of a semi-insulator material on the structure; PA1 e) a step for making an electrical contact on the upper part of the contact layer.
However, one of the most promising applications relates to the monolithic integration of amplifiers with other functions in optics, such as those performed by modulators, change-over switches and distributors. These amplifiers would thus enable compensation for insertion losses and make it possible to set up very complex devices with limited or even zero losses.
One of the major problems with respect to the manufacture of optical amplifiers, whether discrete or integrated, is that of obtaining very low reflection coefficients at the output faces of the amplifiers, in order to avoid a resonant type behavior related to the Fabry-Perot cavity formed by these output faces. Up until now, one of the methods used to obtain low reflection coefficients has been to make anti-reflection dielectric layers that are difficult to control and have little reproducibility.
The following documents describe an approach that enables a reduction in the reflection at the output faces by the making of "window" type structures.
With regard to the integration of an optical amplifier with other optical functions, a recent example consisting of the monolithic integration of three DFB (distributed feed-back) lasers, a multiplexer and an amplifier at output has been described in the following document:
U. KOREN et al.: "An Integrated Tunable Light Source with Extended Tunable Range", I00C'89 (Proceedings), Jul. 18-21, 1989, Kobe, Japan. PA2 an optical confinement layer; PA2 a first optical guiding layer made of a material transparent to the wavelength to be amplified; PA2 a first chemical attack barrier layer; PA2 a second optical guiding layer; PA2 an active layer, the material of which has a maximum gain curve for the energy of the wave to be amplified; PA2 an optical confinement layer; PA2 a contact layer.
The invention relates to a structure including an optical amplifier integrated with a passive waveguide. This structure has made it possible to obtain a fiber-to-fiber gain of the order of 3 to 4 dB, thus demonstrating its potentiel for the making of devices with compensated losses.
Furthermore, this structure has the advantage, in addition to the monolithic integration of two optical functions, of making it possible to obtain very low reflection coefficients at the output faces. For, the structure forming the passive waveguide may be optimized so as to considerably reduce the modal reflection coefficient.
Furthermore, the invention relates to a method for making a new and more promising structure that makes use of the localized epitaxy of semi-insulator layers by MOCVD.
To achieve these results, the invention relates to a method for making an optoelectronic amplifier device comprising the following steps:
The invention also relates to an optical amplifier comprising a waveguide with a forbidden band width greater than the energy of the wave to be amplified as well as a small-sized amplifier element, located on a zone of the waveguide and made of a material for which the maximum value of the gain curve corresponds to the energy of the wave propagated.
Finally, the invention relates to the application of the method to the making of optoelectronic devices.
In particular, it relates to a method for making a modulator applying the above method, the method comprising, before the step for the deposition of the semi-insulator material on the structure, a step for the making of at least one electrode and means for setting up electrical connections to this electrode so as to enable the application of an electrical field to the waveguide.
The invention also relates to a method for making a change-over switch which also implements the above method, the method comprising the making of at least two waveguides including coupling portions and the making, in these coupling portions, of electrodes enabling the application of an electrical field, an amplifier elements being made on at least one of the waveguides.
The invention also relates to a method for making a distributor that also applies the above method, the method comprising the making of several waveguides arranged in a "Y" shape, it being possible to make an amplifier element on each arm of the "Y".